The manufacture of products of the above-mentioned kind has up to now been very complicated and expensive. Today the manufacture is primarily performed by working aluminium, inter alia by high-speed milling and subsequent surface finishing, such as silver-plating, coating, etc. As a result, it is time-consuming to manufacture each component and a great number of manual operations are necessary. Furthermore, it is difficult to obtain the desired dimensional tolerances and quality of the product by this manufacturing process. Thus, as a rule these products need considerable after-treatment.
To solve these problems, the filter casings have, for instance, been provided with trimming means, which allow trimming of the filters after final assembly. However, this makes the filters even more complicated and expensive to manufacture. Moreover, this makes it necessary to test and trim each filter separately by a specialist.
The manufacturing process also significantly limits the possibility of manufacturing certain component parts. High-speed milling allows milling of simple geometric designs only, which makes it necessary to manufacture complicated geometric designs in several pieces, which are subsequently assembled into one functional unit. However, such assembly of several subcomponents into a microwave component almost inevitably leads to a lower degree of dimensional accuracy in the final product, which results in an even greater need for trimming, for instance, of filters after assembly. To arrange trimming means on the filters is time-consuming and considerably increases the costs.
The use of trimming means, such as trimming screws, and the assembly of products from several including parts also constitute a risk of electric disorders, so-called passive intermodulation (PIM). In some applications, this can be disastrous.
The making of the structural or supporting parts of aluminium also limits the thermal dimensional stability and the weight.
As an alternative, it has been proposed in JP 61 079 303 to manufacture waveguides on fusible cores. Around this core, silver and copper are plated, and a carbon fiber fabric is subsequently wound around the core until a thickness of about 2 mm. During the winding, the fabric is impregnated with epoxy resin, and the wound support structure is subsequently cured by supplying heat and pressure, after which the core is melted out. The resulting waveguide consists of a composite structure having continuous carbon fibers with an inner layer of copper and silver.
However, also this manufacturing method suffers from a number of drawbacks. The method is expensive and complicated and requires a great number of manual operations. Thus the method is not suitable for mass production, and the manufacturing time for each component is long and the costs are high.
In addition, the technique is not applicable to the manufacture of filter casings, since it is not possible to wind the carbon fiber fabric in the narrow, downwardly projecting, often circular cavities in the filter casings, or corrugations in horn antennas.
Furthermore, in the prior-art wound carbon fiber waveguide the copper layer cannot affect the rigidity and the thermal stability of the component. In this case, the higher e-module of the carbon fiber structure completely dominates the copper layer, and at temperature changes, which frequently occur in microwave components, this may cause micro-cracking problems in the metal layer. Other problems that may arise are reduced adherence of the composite to the metal and galvanic corrosion due to humidity entering the waveguide through the cracks. The presence of micro-cracks in microwave components, and especially microwave filters, immediately results in reduced electric properties.
It is also a problem with prior-art microwave components that the sensitive electric layer, which internally faces the cavity, often gets damaged either during the manufacturing process or during the use of the component due to different types of environmental influence. This is very serious, since it considerably changes and deteriorates the qualities of the component and usually makes it necessary to replace and discard the component.
Consequently, there is a need for microwave components which can be manufactured at a lower cost and in a more efficient manner, in particular on a large scale, and which also provide better products, have a greater resistance against environmental influence, improved dimensional accuracy, improved thermal dimensional stability, fewer including parts to be integrated and improved electric properties.